A bit more detail...
The first assumption is that Obama wins every State that has voted Dem in each of the last 4 elections. These are California, New York, Illnois, Pennsylvania, Michigan, New Jersey, Massachussets, Washington, Minnesota, Wisconsin, Maryland, Oregon, Connecticut, Hawaii, Maine, Rhode Island, Vermont, Delaware and Washington DC.
The polls I've seen have him ahead in these states. They're basically the States that Kerry won, minus New Hampshire. It would give him 248 of the 270 electoral votes he needs.
On the other side, the Republican bankers should be Texas, North Carolina, Virginia, Indiana, Alabama, South Carolina, Oklahoma, Kansas, Mississippi, Utah, Nebraska, Idaho, Alaska, Wyoming, North Dakota and South Dakota.
Some of these are a lot closer in the polling than Obama's bankers, but the assumption is that they'll come through for McCain. They represent 135 electoral votes.
Of the remaining States, I expect McCain to win Montana, Arizona, Missouri, Arkansas, Louisiana, Tennesee, Kentucky, West Virginia and Georgia. This represents another 78 electoral votes, bringing him up to 213 of the 270 required.
So at this point Obama needs 22 electoral votes and McCain needs 57 and we're left with 7 States and 77 votes in play:
Florida (27)
Ohio (20)
Colorado (9)
Iowa (7)
Nevada (5)
New Mexico (5)
Hew Hampshire (4)
So the scenarios are as follows:
a) Obama takes Florida, wins the election.
b) Obama takes Ohio. Needs any other state (extremely likely) to win the election.
c) McCain wins Florida and Ohio (47 votes). This brings him up to 260 votes, just 10 from victory. Obama still needs 22. The following States are in play:
Colorado (9)
Iowa (7)
Nevada (5)
New Mexico (5)
New Hampshire (4)
In this scenario, Obama would need to take 4 of the 5 States to win. The good news for him is that some polls have him leading in all 5. But I think it's rather optimistic and that McCain will find the 10 he needs from some combination of Colorado, Nevada, New Mexico and possibly even New Hampshire.
The scary possibility is:
Obama takes Colorado + Iowa + one of Nevada or New Mexico
or
Obama takes Iowa + Nevada + New Mexico + New Hampshire
both of which leaves a 269 vs 269 tie...